Ground Beef Casserole

1 1/2 pounds ground beef
1 cup chopped onion
1/2 teaspoon salt
1/2 teaspoon pepper
1/2 cup diced green pepper
1 can (4 ounces) mushrooms, drained
1/2 teaspoon garlic salt
1 can (28 ounces) tomato puree
6 cups cooked spaghetti noodles
1 cup mozzarella cheese

Heat the oven to 350ºF. Brown the beef in a large skillet over medium high heat. Add the onions. Cook and stir until the onions are soft. Stir in the remaining ingredients. Simmer until well blended. Mix with the spaghetti and place in a casserole dish. Top with the mozzarella. Bake for 15 minutes. Servings: 6